The human hip joint acts mechanically as a ball and socket joint, wherein the ball-shaped head of the femur is positioned within the socket-shaped acetabulum of the pelvis. In a total hip joint replacement, both the femoral head and the surface of the acetabulum are replaced with prosthetic devices. One of the critical concerns in a total hip joint replacement procedure is how to achieve a strong attachment between the prosthetic devices and the patient's bone, both at the time of the implantation and throughout the life of the prosthesis. The problem of anchoring the prosthetic devices to a patient's bone is of a particular concern with an acetabular cup prosthesis. Many conventional acetabular cup devices are hemispherical cups which are secured within a prepared acetabulum either with an interference fit, mechanical attachment devices and/or adhesive attachment materials such as bone cement.
The use of bone cement in attaching an acetabular cup prosthesis within an acetabulum provides an excellent immediate attachment but has various disadvantages that appear over time. Load stresses are repeatedly applied to the acetabular cup over the life of the prosthesis. If bone cement is used to secure the cup to the acetabulum, the bone cement may fatigue and fracture under the repeated loading. In some instances, degradation of the bone cement integrity may cause the cup to become loose, thereby necessitating replacement. Additionally, in applying bone cement within a patient's acetabulum, anchoring holes are typically drilled into surrounding bone to provide anchoring points for the bone cement. If the bone cement were to fracture and the cup require replacement, the old bone cement must be removed from the anchoring holes in the bone. Such a procedure is complex, time consuming and potentially destructive to healthy bone structures surrounding the acetabulum. Furthermore, conventional bone cement is cured after it has been dispensed into the patient's acetabulum. Chemical releases occur from the bone cement as the cement is placed and cured within the acetabulum. Such releases may cause adverse reactions in some patients and increases the risks incurred by the patient receiving the hip joint replacement procedure.
Recognizing the disadvantages of cement fixation techniques, prior art acetabular cups have been developed that utilize mechanical attachment means, to join the cup to the acetabulum for immediate stabilization, and various surface treatments intended to bond with bone biologically for long term stable attachment. A simple technique of mechanically securing an acetabular cup, is to affix the cup within the acetabulum with screws or other mechanical fasteners. However, due to the nature of the bone surrounding the acetabulum, and other limiting factors such as artery location and the like, screws can only be applied in certain limited regions. Further, although screws can provide supplemental attachment and stabilization of the cup, they can be used to stably attach a cup where the geometry of the cup is poorly matched to that of the prepared acetabulum.
Another method of mechanically securing an acetabular cup is by the use of threads located on the exterior of the cup. In such an embodiment, the cup is rotated and pushed into the bone of the acetabulum where the teeth of the threads cut into, and engage, the bone. Such a method of implanting an acetabular cup into a patient is exemplified in U.S. Pat. No. 4,662,891 to Noiles, entitled FIXATION ELEMENTS FOR ARTIFICIAL JOINTS. Many prior art cups with threaded exterior surfaces, utilize relatively large threads with sharp points. A disadvantage of such prior art cups is that in the absence of biological attachment between the cup's outer surface and the bone, loading stresses applied to the cup are transferred largely to the threads. Since the periphery of the threads is generally sharp, large stress concentration points are created in the bone in the peripheral region of the threads. Such loading stresses may exceed the amount of stress that can be tolerated by the bone and produce adverse reactions within the bone, which, in some instances results in failure due to cup migration, loosening, pain, and/or joint dislocations.
An alternative method of implanting an acetabular cup involves the use of an interference fit as a means of initial stabilization. Certain prior art devices implanted with an interference fit, employ cups with hemispherical exterior surfaces. The acetabulum is spherically reamed to a given size and an oversized spherical cup is forcibly inserted to provide an interference fit. Spherical reaming of the acetabulum is preferred over other shapes because of its simplicity and ability to be more exactly reproduced from patient to patient. Referring to FIGS. 1a and 1b, a prior art hemispherical acetabular cup 110 is shown in conjunction with a prepared acetabulum 112. The hemispherical acetabular cup 110 has a known radius of curvature. The acetabulum 112 is spherically reamed to a radius of curvature slightly smaller than that of the cup 110. Consequently, the cup 110 can be implanted into the acetabulum 112 with an interference fit. In FIG. 1b, it can be seen that as the cup 110 is driven into the acetabulum 112, the acetabulum 112 is deformed. Only the forces applied by the deformed acetabular surface to the peripheral rim region of the cup 110 have a horizontal holding component which acts to frictionally retain the cup 110 within the acetabulum 112, thereby providing the interference fit. As can be seen in FIG. 1b, the deformation of the acetabulum results in a gap 113 between a surface region of the cup and of the acetabulum. No forces are applied to those portions of the cup 110 surrounded by the gap 113. Below the gap, however, the cup 110 once again engages the surface of the acetabulum, this time at the most apical region, where the vertical reactive forces imparted to the cup thereby tend to produce a bounce back effect that may prevent the cup 110 from being fully seated.
Other prior art acetabular cups have been developed that control the region of interference between the cup and the acetabulum, produced by an interference fit implantation. Referring to FIGS. 2a and 2b, a cup 114 is shown that has an external surface with two radii of curvature. The region 115 proximate the rim of the cup 114 has a larger radius of curvature than does the apical region 116 of the cup 114. The cup 114 is conventionally inserted into an acetabulum 117 that has been reamed to a radius of curvature approximately equal to the apical region 116 of the cup 114. When the cup 114 is fit within the acetabulum 117, the region 115 proximate the rim of the cup 114 displaces the acetabulum 117 and creates an interference fit. The displacement of the acetabulum 117 by the cup 114 causes the acetabulum 117 to deform away from its original spherically reamed shape. Consequently, the bottom of the reamed acetabulum is no longer spherically shaped as it is contacted by the apical region 116 of the cup 114. As can be seen from FIG. 2b, the spherically curved apical region 116 of the cup 114 does not perfectly conform to the non-spherical bottom of the acetabulum 117. Consequently, grooves 118 may exist along the cup-to-bone interface. A prior art cup embodying a dual-radius as described is exemplified in U.S. Pat. No. 4,892,549 to Figgie, et al., entitled DUAL-RADIUS ACETABULAR CUP COMPONENT.
In FIGS. 3a and 3b, a different prior art embodiment is shown wherein the acetabular cup 120 has a spherically curved apical region 121 and a frustrum-shaped rim region 122. The difference in shapes between the rim region 122 and the apical region 121 is pronounced, providing the cup 120 with a stepped exterior surface. To accommodate the cup 120, the acetabulum 124 must be reamed with two differently sized and shaped reamers so that the acetabulum 124 can properly accommodate the stepped exterior of the cup 120. The required two stepped reaming operation of acetabulum 124 increases the complexity and labor required in implanting the cup 120. As the cup 120 is fit within the reamed acetabulum 124, the rim region 122 of the cup 120 displaces the acetabulum 124 so as to create an interference fit between the cup rim region 122 and the acetabulum 124. The acetabulum 124 is originally reamed to be spherical. However, the displacement of the acetabulum 124 caused by the rim region 122 of the cup 120, causes the acetabulum 124 to distort away from its original spherical shape. Consequently, the spherically shaped apical region 121 of the cup 120 does not lay flush against the acetabulum 124. As such, grooves 125 may occur along the cup-to-bone interface at various positions across the apical region 121 of the cup 120. A prior art cup embodying the cup geometry as above-described is exemplified in U.S. Pat. No. 4,704,127 to Averill, et al., entitled DUAL-GEOMETRY ACETABULAR CUP COMPONENT AND METHOD OF IMPLANT.
It is also noted that certain prior art acetabular cups are provided to the users as a two-part device, wherein an inner bearing insert is fitted to an outer shell by the practitioner. Employing a two-part device makes it possible to offer one series of outer shell sizes and to provide an array of inner bearing inserts for subsequent assembly that accommodate differently sized femoral heads. A two-part construction also allows the femoral head to contact a material that provides less friction to the femoral head than would the material of the shell. Such a two-part cup prosthesis is exemplified by U.S. Pat. No. 4,795,470 to Goymann et al., entitled, TWO-PART SOCKET FOR HIP-JOINT PROSTHESIS.
It is known that when a patient with a hip joint replacement implements motions of that limb, the head of the femoral prosthesis will transmit forces to the cup in varying directions and that the neck of the femoral prosthesis may occasionally contact the rim of the implanted cup. As a result of such varying forces and/or contact, forces to occur which attempt to move the cup relative to the acetabulum in various ways. In the two piece cup described above, these same complex forces act on the inner bearing and attempt to rock, rotate and translate the inner bearing insert relative to the outer shell. A less than fully secured bearing insert would result in micro-motion between the bearing insert and the outer shell. Such micro-motion may cause wear of the bearing insert which further detracts from the integrity of the insert-to-shell interface. To prevent the disassociation of the insert from the shell, various methods of retaining the bearing insert within the outer shell have been developed. For example, certain prior art devices have a keyed projection formed on the outer surface of the bearing insert. The keyed projection fits within an aperture, formed through the outer shell, to assure the general apical alignment of the bearing insert relative to the outer shell, but like other commercialized methods of retention, does not prevent micro-motion between shell and insert. Such prior art prosthetic devices arc exemplified by U.S. Pat. No. 4,878,916 to Rhenter et al. entitled PROSTHETIC CUP.
Another common prior art method or affixing a bearing insert within a shell component is by using screws or other mechanical fasteners, as exemplified by U.S. Pat. No. 5,092,897 to Forte, entitled IMPLANTABLE ACETABULAR PROSTHETIC HIP JOINT WITH UNIVERSAL ADJUSTABILITY. This construction provides security against the macro-motion of the acetabular cup within the acetabulum but generally does not prevent the micro-motion that occurs at the screw head to shell interface or at the insert-to-shell interface.
Another prior art method of retaining a bearing insert within the shell component is by using a snap-fit configuration. Such mechanisms have addressed, to varying degrees, the potential for disassociation of the insert from the shell but do not address the potential for micro-motion and the resulting adverse affects of wear debris.
Two pan acetabular cups are intended to be assembled at the time of surgery for the convenience of insert size selection and outer shell insertion. This manual assembly during surgery requires that the manufacturers insure the long term interchangeability of parts, which in turn leads to dimensional tolerances that produce clearances between the two components in most, if not all assemblies. These designed clearances result in an unavoidable potential for micro-motion existing between the insert and the shell when exposed to the complex and cyclic loads that are predictably experienced by the hip joint. Furthermore, these clearances between the parts constitute void spaces that fill with body fluids and mix with the wear debris that result from micro-motion. Since the loads being transmitted across the two parts are both cyclic and varied in direction, the void spaces open and close in a pistoning fashion causing the contained mixture of body fluids and wear debris to be expressed. If apertures exist in the shell in the form of screw holes or other apertures, the debris laden fluids may be expressed into the interface between the shell and the acetabulum where fixation between the shell and the bone is intended. The injection of wear debris into the bone is clearly undesirable in light of the fact the wear debris is thought to cause bone lysis which in turn may lead to loosening of the cup and the need for reoperation with reduced probability of an enduring reconstruction.
In view of the above, there is described an improved acetabular cup that can be implanted with an interference fit and is operative to avoid many of the abovenoted prior art problems relating to the fit and stability of the cup. There is further described an improved shell component and bearing insert that is preassembled in a manner that substantially eliminates both micro-motion and voids between the shell component and the bearing insert.